Many plastics mouldings, such as plastics bottles and medical and automotive mouldings, are produced by means of an extrusion blow moulding machine which forms a plastics tube, called a parison, by extrusion, and then, after transferring it to a mould, inflates the tube within the mould. In use of such an extrusion blow moulding technique in a production process, changes in the constituency of the moulding material or in the ambient temperature, or machine variations can all have an effect on the properties of the final moulded product, and in particular on the weight of the product. Since variation in the product weight can cause it to drift outside acceptable limits, it is important to control the extrusion blow moulding process so as to keep the product weight within acceptable limits.
A conventional extrusion blow moulding machine incorporates an annular die gap through which the moulding material is extruded to produce the tubular parison which is subsequently subjected to blow moulding to produce a product which is outputted from the machine. The die gap can be controlled by a movable member which is displaceable by supply of hydraulic fluid to a cylinder under the control of an electrohydraulic servovalve. The servovalve is under the control of a parison programmer which is set by the machine operator to the desired wall thickness and weight settings and which receives a feedback signal from a transducer indicative of the actual position of the movable member, and which supplies an output signal to the servovalve to effect movement of the movable member to a desired position corresponding to the desired width of the die gap. Furthermore the parison programmer may be programmed to vary the wall thickness during extrusion of the parison so that the wall thickness of the generally tubular parison varies along its length in the manner required by the form of the final moulded product, that is to take account of features such as the shaped bottom and the necked outlet of a container, for example.
Whilst such a control system enables the parison wall thickness to be controlled so as to produce final moulded products of reasonable quality and within a permitted weight range, the inherent tolerances of the control system are such that it may be necessary for the machine operator to aim to produce overweight products so as to ensure that the lightest product so produced does not fall below the permitted minimum weight.
It is an object of the invention to provide a novel interface unit for an extrusion blow moulding control system which enables parison wall thickness to be controlled with sufficient accuracy to enable the weight of the final product to be maintained within a narrow weight range.